We have reported that a radioimmunoassay based on a synthetic analog of the amino terminus of human prolactin, hPRL(1-12)TyrOH, is suitable for measuring hPRL in plasma. The orientation of the hapten is such that hPRL(2-12)TyrOH lost 1/3 of the displacement activity; hPRL(3-12)TyrOH, 2/3; and hPRL(4-12)TyrOH was inactive. The 13 residue analog iodinates well and is suitable for use as a label for at least 10 weeks, and interassay variability appears to be slight. The finding in our laboratories that hPRL undergoes glycosylation poses new problems to measurement now being addressed. A technique based on immunoprecipitation of hormones from plasma employing first and second antibody, followed by gel electrophoresis, immunoblotting, and radiographic demonstration of hormonal bands has permitted the demonstration of the 20 kilodalton hGH variant in 1 or 2 ml of normal plasma. Further, while looking for cleaved forms of growth hormone in bovine pituitary glands, we found a fraction that was fully active in the tibial line assay for GH. It was devoid of immunoactivity in RIAs for bGH, for IGF-I, and F.G.F. A chondrocyte cell culture has speeded isolation work, and enhanced thymidine uptake (20-\to 30-fold) gives excellent correlation with the tibial line test. We consider this "tibial growth factor" to be a strong candidate for the bioactive/immunologically inactive GH of Ellis. It may account for rapid growth in the absence of hGH (by RIA) in chidren post-operative for craniopharyngioma. Studies of the amino terminus of hGH indicate that residues 1-43 enhance the binding of insulin to hepatic receptors from female rats, especially. (1)